Fast food generally costs very little, which is one of the perks, but that doesn't mean a free pizza or burger isn't enticing if all you have to do is fill out a feedback form. According to Redditor princesskiki, it's that easy to receive coupons in person and in the mail:

Many restaurants will give out coupons for free food in return for feedback on their restaurant. They are desperate for feedback and have customer suppiort departments that will take care of people who give them the feedback they need. To get ahold of these coupons, you simply go online to their website and look for the "contact" or "feedback" links that most of them have. Fill out the forms with the true details of your experience, and take a minute and give them good solid feedback. Usually you will be required to enter personal information including your address and/or phone number and this is how you will receive your free food coupons. Sometimes they WILL call you to talk to you about your experience. Don't worry..they don't bite!

Whether or not giving feedback is worth your time is up to you, but princesskiki says she's received a stack of free burger coupons from Carl's Jr., a free pizza from Pizza Hut, free value meals from In and Out, and much more just for filling out a comment card. In some cases, it even worked at regular chain restaurants. It's worth a shot if you don't want to pay.